From West German publication DE-OS No. 23 26 810, a 3 dB directional coupler is known which includes two parallel coupling lines in the form of strip lines connecting respective inner conductors of coaxial lines and arranged in spaced-apart relationship so as to define a gap therebetween. For controlling the coupling attenuation, the gap is adjusted by pins which engage the center of the coupling path. This directional coupler which is preferably used for power combination or power distribution is not suitable when transmission of considerable power at very short waves is required. When in view of the power to be transmitted the cross section of the line is to be provided of such magnitude that the critical frequency of the line is only slightly above the operating frequency, the coupling path cannot be accurately defined anymore because the extensive dimensions of the strip conductors require in comparison to the operating wave length considerable radii at the junction to the coupling path as will be explained by the following example:
A coaxial line with a wave impedance of 50.OMEGA. which is dimensioned for a frequency of 800 MHz and for the maximum transmittable power at this frequency has an outer conductor with a diameter of about 150 mm and an inner conductor with a diameter of 63 mm. The coupling path of the directional coupling has a length of .lambda./4 of about 93 mm wherein .lambda. is the wavelength at the design frequency.
Such a ratio of the diameter of the inner conductor to the length of the coupling path results in a directional coupler of the above-mentioned kind in which inevitably field distortions and field inhomogeneities occur at the junction of the respective inner conductor of the coaxial line with the strip conductor extending in the area of the coupling path so that the directivity and the frequency response of the directional coupler is deteriorated. A further problem of this known coupler is the obtained bending of the strip conductors when adjusting the coupling attenuation by shifting the pins in axial direction thereof so that parallelism of the strip conductors is not maintained along the coupling path.